Existing tool like "msgfmt --check" can check typical translation
problems (missing "%s" in msgstr, for example), but can't check
Mercurial specific ones.
For example, "msgfmt --check" can't check whether the translated
string given to "ui.promptchoice()" is correct or not, even though
problems like below cause run-time error or unexpected behavior:
- less or more choices than msgid,
- choices without '&', or
- choices with '&' followed by none
This patch adds the tool to check Mercurial specific translation
problems in *.po files.
